1. Field of the Invention
The present invention relates to an audio signal processing device and an audio signal processing method performing audio signal processing for acoustically reproducing audio signals of two or more channels such as signals for a multi-channel surround system by electro-acoustic reproduction means for two channels arranged close to both ears of a listener. Particularly, the invention relates to the audio signal processing device and the audio signal processing method allowing the listener to listen to the sound as if sound sources virtually exist at previously assumed positions such as positions in front of the listener when the sound is reproduced by electro-acoustic transducer means such as drivers for acoustic reproduction of, for example, headphones, which are arranged close to the listener's ears.
2. Description of the Related Art
For example, when the listener wears headphones at the head and listens to an acoustic reproduction signal by both ears, there are many cases where the audio signal reproduced in the headphones is a normal audio signal supplied to speakers set on right and left in front of the listener. In such case, it is known that a phenomenon of so-called inside-the-head localization occurs, in which a sound image reproduced in headphones is shut inside the head of the listener.
As a technique addressing the problem of inside-the head localization problem, a technique called virtual sound image localization is disclosed in, for example, WO95/13690 (Patent Document 1) and JP-A-3-214897 (Patent Document 2).
The virtual sound image localization is the technique of reproducing sound as if sound sources, for example, speakers exist at previously assumed positions such as right and left positions in front of the listener (sound images are virtually localized at the positions) when the sound is reproduced by headphones and the like, which is realized as follows.
FIG. 29 is a view for explaining a method of the virtual sound image localization when reproducing a right-and-left 2-channel stereo signal by, for example, 2-channel stereo headphones.
As shown in FIG. 29, microphones ML and MR are set at positions (measurement point positions) close to both ears of the listener at which two drivers for acoustic reproduction of, for example, the 2-channel stereo headphones are assumed to be set. Additionally, speakers SPL, SPR are arranged at positions where the virtual sound images are desired to be localized. Here, the driver for acoustic reproduction and the speaker are examples of the electro-acoustic transducer means and the microphone is an example of an acoustic-electric transducer means.
First, acoustic reproduction of, for example, an impulse is performed by a speaker SPL of one channel, for example, a left channel in a state in which a dummy head 1 (or may be a human being, namely, a listener himself/herself) exists. Then, the impulse generated by the acoustic reproduction is picked up by the microphones ML and MR respectively to measure a head related transfer function for the left channel. In the case of the example, the head related transfer function is measured as an impulse response.
In this case, the impulse response as the head related transfer function for the left channel includes an impulse response HLd of a sound wave from the speaker for the left channel SPL (referred to as an impulse response of left-main component in the following description) picked up by the microphone ML and an impulse response HLc of a sound wave from the speaker for the left channel SPL (referred to as an impulse response of a left-crosstalk component) picked up by the microphone MR as shown in FIG. 29.
Next, acoustic reproduction of an impulse is performed by a speaker of a right channel SPR in the same manner, and the impulse generated by the reproduction is picked up by the microphones ML, MR respectively. Then, a head related transfer function for the right channel, namely, the impulse response for the right channel is measured.
In this case, the impulse response as the head related transfer function for the right channel includes an impulse response HRd of a sound wave from the speaker for the right channel SPR (referred to as an impulse response of a right-main component in the following description) picked up by the microphone MR and an impulse response HRc of a sound wave from the speaker for the right channel SPR (referred to as an impulse response of a right-crosstalk component) picked up by the microphone ML.
Then, the impulse responses as the head related transfer function for the left channel and the head related transfer function for the right channel which have been obtained by measurement are convoluted with audio signals supplied to respective drivers for acoustic reproduction of the right and left channels of the headphones. That is, the impulse response of the left-main component and the impulse response of the left-crosstalk component as the head related transfer function for the left channel obtained by the measurement are convoluted as they are with the audio signal for the left channel. Also, the impulse response of the right-main component and the impulse response of the right-crosstalk component as the head related transfer function for the right channel obtained by the measurement are convoluted as they are with the audio signal for the right channel.
According to the above, in the case of, for example, the right and left 2-channel stereo audio, the sound image can be localized (virtual sound image localization) as if the sound is reproduced at the right-and-left speakers set in front of the listener though the sound is reproduced near the ears of the listener by the two drivers for acoustic reproduction of the headphones.
The above is the case of two channels, and in the case of multi channels of three channels or more, speakers are arranged at virtual sound image localization positions of respective channels and, for example, an impulse is reproduced to measure head related transfer functions for respective channels in the same manner. Then, the impulse responses as the head related transfer functions obtained by measurement may be convoluted with audio signals to be supplied to the drivers for acoustic reproduction of right-and-left two channels of the headphones.
Recently, the multi-channel surround system such as 5.1-channel, 7.1-channel is widely used in sound reproduction when video of DVD (Digital Versatile Disc) is reproduced.
It is also proposed that the sound image localization in accordance with respective channels (virtual sound image localization) is performed by using the above method of the virtual sound image localization also when the audio signal of the multi-channel surround system is acoustically reproduced by the 2-channel headphones.